It is known in the art that the formation of an oil-bearing zone can be fractured by subjecting it to a very high pressure. This is usually done by injecting a fracturing liquid, such as a brine, an acid, or a micellar solution into the borehole, and increasing the pressure to a value at which the rock fractures. Propping agents can be added to the fracturing liquid which migrate into the fissures of the rock and keep them open after releasing the pressure. It also has been proposed to increase the viscosity of fracturing fluids by incorporating soluble polymers in the fracturing fluids. The increased viscosity is desirable both for reducing losses of the fluid into highly permeable zones and for increasing the capability of the fracturing fluid to carry solid propping agents.
It would be desirable to have a fracturing fluid available which is initially viscous but whose viscosity can be controllably reduced over a given period of time. Thus one desires a fluid that remains highly viscous just slightly longer than the fracturing period and thereafter becomes sufficiently liquid or fluid to be easily displaced from the newly formed fissures by the oil and readily pumped out of the borehole.
Another field of the oil recovery art where viscosificiation of fluids is important is that of drilling. In certain operations, particularly where relatively thin drilling fluids such as water or brine are used, it is periodically desirable to clean the borehole and to remove cuttings therefrom. This can be achieved by circulating a composition through the well, which is viscous and which can carry cuttings out of the borehole. In this instance, too, it would be desirable to have a composition that has high viscosity, but loses this viscosity in a controlled time schedule. The viscous composition would then be adjusted so that it loses the high viscosity shortly after the cleaning cycle has been finished so that it can be conveniently displaced from the borehole by the thinner drilling fluid when drilling is resumed.